Bitstream conversion method, bitstream conversion apparatus, bitstream connecting apparatus, bitstream splitting program, bitstream conversion program, and bitstream connecting program

ABSTRACT

A bitstream conversion apparatus for converting a bitstream of a first format, containing content data, into a bitstream of a second format includes: a splitting unit which splits the bitstream of the first format in time sequential fashion into a plurality of split bitstreams of the first format; a plurality of conversion units which convert the plurality of split bitstreams of the first format into a plurality of split bitstreams of the second format; and a connecting unit which connects the plurality of split bitstreams of the second format to one another.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation application based onInternational application No. PCT/JP2007/055556, filed on Mar. 19, 2007.

TECHNICAL FIELD

The present invention relates to media signal encoding techniques forcompressing moving image signals and audio signals by utilizing spatialand temporal correlation inherently present in the moving image signalsand temporal correlation inherently present in the audio signals. Moreparticularly, the invention relates to stream conversion techniques forconverting a bitstream containing a moving image stream and an audiostream from one format to another.

BACKGROUND ART

For providers who deliver bitstreams containing content data, such asmoving image streams and audio streams, if the efficiency of networkresources can be enhanced, it will be advantageous in reducingoperational costs. There is therefore a need to reduce the file size ofdata to be delivered, by converting already compressed bitstreams intobitstreams encoded by a higher-efficiency encoding scheme.

From a user's side there is also a need for converting a large-capacityvideo stream into a video stream of a smaller file size so that thevideo stream can be easily previewed on an existing network having alimited transmission band.

FIGS. 1A and 1B are diagrams each depicting a prior art process forconverting a bitstream encoded in a given format (scheme A) into abitstream of a different format (scheme B). The process depicted in FIG.1A performs the conversion by using a decoder of scheme A, whichconverts the bitstream of scheme A back into the original data, incombination of an encoder of scheme B which converts the original datainto the bitstream of scheme B, while the process depicted in FIG. 1Bconverts the bitstream of scheme A directly into the bitstream of schemeB. In either case, the conversion process is not performed in a parallelfashion, but performed continuously starting from the beginning of thebitstream and working through to the end thereof.

The bitstream format is defined by using various elements. For example,when the bitstream contains a moving image stream, the bitstream formatis defined, for example, by the encoding scheme used to encode themoving image stream, the bit rate and frame rate of the stream or thesize of the moving image, etc. On the other hand, when the bitstreamcontains an audio stream, for example, the bitstream format is definedby the encoding scheme used to encode the audio stream, the bit rate andframe rate of the stream, etc.

Patent document 1 listed below discloses a video encoding method andapparatus that can encode video scene data efficiently in a parallelfashion.

Patent document 2 listed below discloses an image encoding and decodingmethod which, when an image is encoded after it is split into aplurality of screen images, can prevent boundaries from becomingnoticeable in the reconstructed image.

Patent document 3 listed below discloses a moving image encodingapparatus that can encode a moving image signal with high efficiencywhile preventing image degradation even when the moving image signal isencoded after it is divided it into a plurality of screen images.

Patent document 4 listed below discloses a method for using the slicestructure used in video encoding in order to encode and decode a part ofvideo redundantly.

Patent document 5 listed below discloses a multimedia conversion systemthat converts only the difference between the section the user specifiedfrom among multimedia data and the already converted section of themultimedia data and combines it with the already converted multimediadata for delivery to the user.

Patent document 6 listed below discloses an encoding method that makesit possible to reproduce an image smoothly even when a plurality ofseparately encoded bitstreams are decoded successively.

Patent document 7 listed below discloses a video/audio signal editingapparatus which, when trimming an MPEG-2PS stream, generates datacontaining sequence-end information that follows the data immediatelypreceding the deletion start position, generates data containingsequence-start information at the position immediately preceding thedata that follows the deletion end position, and updates the filemanagement information so that the generated data are contiguous witheach other.

Patent document 8 listed below discloses a distributed paralleltranscoder system, which limits the connecting point of a plurality ofsplit image data to a scene change point.

Patent document 1: Japanese Unexamined Patent Publication No.2002-199392

Patent document 2: Japanese Unexamined Patent Publication No. H09-284756

Patent document 3: Japanese Unexamined Patent Publication No. H05-183891

Patent document 4: Japanese Unexamined Patent Publication No.2004-236337

Patent document 5: Japanese Unexamined Patent Publication No.2006-352443

Patent document 6: International Publication Pamphlet No. 97/13367

Patent document 7: Japanese Unexamined Patent Publication No. 2005-33382

Patent document 8: Japanese Unexamined Patent Publication No.2005-176069

DISCLOSURE OF THE INVENTION

As previously described, the need for converting a bitstream of a givenformat into a bitstream of a different format has been greatlyincreasing, and the number of pieces of content that need suchconversion has also been rapidly increasing. However, the conversion ofa bitstream (especially, in the case of a moving image stream) involvesa large amount of processing and, given the present state of the art ofcomputers commonly in use today, the ratio of conversion time to streamplayback time cannot be readily increased. As a result, according to theprior art method that does not perform parallel processing, if abitstream having a long playback time is to be converted, the requiredtime inevitably increases.

In view of the above situation, it is an object of the present inventionto reduce the bitstream conversion time by splitting the bitstream andconverting the split bitstreams in a parallel fashion.

In the present invention, when converting a bitstream of a first format,containing content data, into a bitstream of a second format, thebitstream of the first format is first split in time sequential fashioninto a plurality of split bitstreams of the first format, and then thesesplit bitstreams are converted in a parallel fashion into a plurality ofsplit bitstreams of the second format; after that, the split bitstreamsof the second format are connected together to produce the bitstream ofthe second format.

According to the present invention, since the conversion process thatinvolves the largest amount of processing can be performed in a parallelfashion, it is possible to reduce the conversion time by increasing thenumber of bitstream conversion means.

However, when the plurality of bitstreams produced after a singlebitstream is split in time sequential fashion are converted in aparallel fashion, since each stream is converted without usinginformation from its immediately preceding stream, a substantialdifference in image quality can occur at each stream connectingposition, rendering the resulting image visually unbalanced.

Further, in many bitstream encoding techniques, a speed smoothing memorycalled a buffer is used to smooth out variations in encoded signaltransmission speed caused by variations in the amount of producedinformation, and an upper limit to the buffer occupancy, whichrepresents the amount of information that can be stored in the buffer,is predefined in a specification. During encoding, overflow control isperformed by controlling the bit rate for encoding so that the upperlimit will not be exceeded. However, if a single bitstream is split intoa plurality of bitstreams, and the streams are converted in a parallelfashion, then when performing overflow control during the encoding ofany given stream it is not possible to refer to the amount ofinformation that occurred during the encoding of the previous stream. Asa result, when the bitstreams are connected together, a situation canoccur where the buffer occupancy exceeds the predefined upper limit.

In view of this, in the present invention, provisions are made to splitthe bitstream of the first format by providing an overlapping portion ateach splitting position so that a selection can be made from a pluralityof splitting positions.

Further, in the present invention, prescribed auxiliary information thatis referred to in order to select a connecting position at which toconnect together the split bitstreams of the second format is generatedfor each key frame contained in the overlapping portion of the splitbitstreams of the second format. Then, the split bitstreams of thesecond format are connected together at the key frame position where thedifference between the auxiliary information generated for each ofsuccessive bitstreams are connected together is the smallest.

For example, when the bitstream of the first format contains an imagestream, information concerning the amount of information produced duringthe processing of each frame contained in the image stream, informationconcerning the image quality of each frame, information that provides ameasure of similarity between frames, etc., may be used as the auxiliaryinformation.

When information concerning the amount of information and image qualityis used as the auxiliary information, and when any two successivestreams are connected together at the position where the auxiliaryinformation difference between the corresponding key frames contained inthe overlapping portion of the streams is the smallest, the differencein image quality at the connecting position becomes substantiallyunnoticeable.

Further, buffer occupancy information such as VBV (Video BufferingVerifier) buffer occupancy, i.e., the amount of information obtained bysubtracting, from the total amount of information produced during theencoding of the first frame to the current frame, the amount ofinformation that can be transmitted for playback during that period, maybe used as the amount of produced information.

When such buffer occupancy information is used as the auxiliaryinformation, and when the streams are connected together at the positionwhere the difference in buffer occupancy is the smallest, it is possibleto suppress the variation in buffer occupancy that occurs at theconnecting position, and thus the buffer occupancy can be prevented fromviolating the predefined upper limit.

Furthermore, when information that provides a measure of similaritybetween corresponding frames, such as motion vector informationgenerated for the respective frames, is used as the auxiliaryinformation, and when the streams are connected together at the positionwhere the difference of such information is the smallest, the differencein image quality at the connecting position can also be madesubstantially unnoticeable.

According to a first mode of the present invention, there is provided abitstream conversion method for converting a bitstream of a firstformat, containing content data, into a bitstream of a second format.The method includes: splitting the bitstream of the first format in timesequential fashion into a plurality of split bitstreams of the firstformat; converting the plurality of split bitstreams of the first formatin a parallel fashion into a plurality of split bitstreams of the secondformat; and connecting the plurality of split bitstreams of the secondformat to one another.

According to a second mode of the present invention, there is provided abitstream conversion apparatus for converting a bitstream of a firstformat, containing content data, into a bitstream of a second format.The bitstream conversion apparatus includes: a splitting unit whichsplits the bitstream of the first format in time sequential fashion intoa plurality of split bitstreams of the first format; a plurality ofconversion units which convert the plurality of split bitstreams of thefirst format into a plurality of split bitstreams of the second format;and a connecting unit which connects the plurality of split bitstreamsof the second format to one another.

According to a third mode of the present invention, there is provided abitstream connecting apparatus for connecting a plurality of splitbitstreams of a second format to one another, the plurality of splitbitstreams of the second format being obtained by first splitting abitstream of a first format, containing content data, in time sequentialfashion into a plurality of split bitstreams of the first format so asto provide an overlapping portion therebetween and then converting thesplit bitstreams of the first format into the split bitstream of thesecond format. The bitstream connecting apparatus includes a connectingframe position determining unit and a data connecting unit whichconnects one stream to an ensuing stream at a connecting frame position;the connecting frame position determining unit receives prescribedauxiliary information that is generated for each key frame contained inthe overlapping portion of the split bitstreams of the second format andthat is referred to in order to select the connecting position at whichto connect together the split bitstreams of the second format. Then, theconnecting frame position determining unit determines the connectingframe position for connecting one stream to the ensuing stream byselecting a key frame position where the difference between theauxiliary information generated for the one stream and the auxiliaryinformation generated for the ensuing stream is the smallest.

According to a fourth mode of the present invention, there is provided abitstream splitting program for use in a bitstream conversion apparatusthat converts a bitstream of a first format, containing content data,into a bitstream of a second format, and that includes a splitting unitwhich splits the bitstream of the first format in time sequentialfashion into a plurality of split bitstreams of the first format, aplurality of conversion units which convert the plurality of splitbitstreams of the first format into a plurality of split bitstreams ofthe second format, and a connecting unit which connects the plurality ofsplit bitstreams of the second format to one another, the bitstreamsplitting program being run on a computer for bitstream splittingprocess provided as the splitting unit. This bitstream splitting programcauses the computer for betstream splitting process to execute asplitting process for splitting the bitstream of the first format intime sequential fashion into the plurality of split bitstreams.

According to a fifth mode of the present invention, there is provided abitstream conversion program which is run on a computer for bitstreamconversion process provided as the conversion unit that converts, intothe split bitstreams of the second format, the split bitstreams of thefirst format into which the bitstream of the first format, containing atleast a moving image stream, has been split by the bitstream splittingprogram of the fourth mode described above. This bitstream conversionprogram causes the computer for bitstream conversion process to executean auxiliary information generating process for generating, for each keyframe contained in the overlapping portion of the split bitstreams ofthe second format, prescribed auxiliary information that is referred toin order to select a connecting position at which to connect togetherthe split bitstreams of the second format.

According to a sixth mode of the present invention, there is provided abitstream connecting program which is run on a computer for bitstreamconnecting process provided as the connecting unit that receives fromthe plurality of conversion units the auxiliary information and thesplit bitstreams converted to the second format by the bitstreamconversion program of the fifth mode described above, and that connectsthe plurality of split bitstreams of the second format to one another.This bitstream connecting program causes the computer for bitstreamconnecting process to execute: a connecting position determining processfor determining a connecting position for connecting one stream to anensuing stream by selecting a key frame position where the differencebetween the auxiliary information generated for the one stream and theauxiliary information generated for the ensuing stream is the smallest;and a data connecting process for connecting the one stream to theensuing stream at the thus determined connecting position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram (part 1) illustrating a prior art process forconverting a bitstream encoded in a given format (scheme A) into abitstream of a different format (scheme B).

FIG. 1B is a diagram (part 2) illustrating a prior art process forconverting a bitstream encoded in a given format (scheme A) into abitstream of a different format (scheme B).

FIG. 2 is a schematic diagram illustrating the generation configurationof a bitstream conversion apparatus according to an embodiment of thepresent invention.

FIG. 3 is a schematic diagram illustrating the configuration of asplitting unit depicted in FIG. 2.

FIG. 4 is a schematic diagram illustrating the configuration of aconversion unit depicted in FIG. 2.

FIG. 5 is a schematic diagram illustrating the configuration of aconnecting unit depicted in FIG. 2.

FIG. 6 is a diagram illustrating a functional block diagram implementedby the splitting unit depicted in FIG. 2.

FIG. 7 is a flowchart illustrating a first example of a bitstreamsplitting position determining method according to the presentinvention.

FIG. 8A is an explanatory diagram (part 1) for the bitstream splittingposition determining method according to the present invention.

FIG. 8B is an explanatory diagram (part 2) for the bitstream splittingposition determining method according to the present invention.

FIG. 9 is a flowchart illustrating a second example of the bitstreamsplitting position determining method according to the presentinvention.

FIG. 10 is a diagram illustrating a first example of a functional blockdiagram implemented by the conversion unit depicted in FIG. 2.

FIG. 11 is a diagram illustrating a first example of a functional blockdiagram implemented by the connecting unit depicted in FIG. 2.

FIG. 12 is a flowchart illustrating a first example of a bitstreamconnecting method according to the present invention.

FIG. 13 is an explanatory diagram for the bitstream connecting methodillustrated in FIG. 12.

FIG. 14 is a flowchart illustrating a second example of the bitstreamconnecting method according to the present invention.

FIG. 15 is an explanatory diagram for the bitstream connecting methodillustrated in FIG. 14.

FIG. 16 is a diagram illustrating a second example of a functional blockdiagram implemented by the conversion unit depicted in FIG. 2.

FIG. 17 is a diagram illustrating a second example of a functional blockdiagram implemented by the connecting unit depicted in FIG. 2.

FIG. 18 is an explanatory diagram illustrating how a conversion isredone according to the present invention.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1 . . . BITSTREAM CONVERSION APPARATUS    -   h1, h2, hn . . . AUXILIARY INFORMATION    -   S . . . BITSTREAM OF SECOND FORMAT    -   S1, S2, Sn . . . SPLIT BITSTREAMS OF SECOND FORMAT    -   tx1, tx2, txn . . . OVERLAPPING PORTIONS    -   V . . . BITSTREAM OF FIRST FORMAT    -   V1, V2, Vn . . . SPLIT BITSTREAMS OF FIRST FORMAT

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to the accompanying drawings. FIG. 2 is a schematic diagramillustrating the generation configuration of a bitstream conversionapparatus according to the embodiment of the present invention. Thebitstream conversion apparatus 1 is an apparatus for converting abitstream V of a first format, containing a moving image stream andaudio stream as content, into a bitstream S of the same content but of asecond format by converting the format of the bitstream. The differencein format between the bitstream V of the first format and the bitstreamS of the second format may lie, for example, in the encoding scheme usedto encode the stream, the image size, or the bit rate or frame rate ofthe stream.

The bitstream conversion apparatus 1 includes a splitting unit 10 whichsplits the bitstream V of the first format in time sequential fashioninto a plurality of split bitstreams v1 to vn of the first format, aplurality of conversion units 20 a to 20 n which convert the pluralityof split bitstreams v1 to vn of the first format into a plurality ofsplit bitstreams s1 to sn of the second format, and a connecting unit 30which connects the plurality of split bitstreams s1 to sn of the secondformat to produce the bitstream S of the second format having the samecontent as the bitstream V of the first format but different in format.In the following description, the plurality of conversion units 20 a to20 n may sometimes be referred to collectively as the conversion unit20.

The diagram of the bitstream V depicted in FIG. 2 indicates that theplurality of split bitstreams v1 to vn of the first format, produced bythe splitting unit 10, have overlapping portions between them; forexample, the first split bitstream v1 and the second split bitstream v2have an overlapping portion tx1 between them, the second split bitstreamv2 and the third split bitstream (not depicted) have an overlappingportion tx2 between them, and the (n−1)th split bitstream (not depicted)and the n-th split bitstream vn have an overlapping portion txn−1between them.

FIG. 3 is a schematic diagram illustrating the configuration of thesplitting unit 10 depicted in FIG. 2. The hardware configuration of thesplitting unit 10 can be implemented using a computer. That is, thesplitting unit 10 includes a processor 11 such as a CPU, a storage unit12 which stores an operating system (OS) 13 and other programs to beexecuted by the processor 11, a memory 15 into which programs to beexecuted by the processor 11 are loaded and in which data necessary forprocessing by the processor 11 are temporarily stored, and an interface(I/O) 16 which receives the bitstream V of the first format from theoutside and outputs the split bitstreams v1 to vn of the first format tothe respective conversion units 20 a to 20 n.

The storage unit 12 stores a splitting program 14 which is executed bythe processor 11 and thereby causes the processor 11 to carry out thesplitting process for splitting the input bitstream V of the firstformat into the split bitstreams v1 to vn of the first format. Thedetails of the bitstream splitting process implemented by the splittingprogram 14 will be described later.

FIG. 4 is a schematic diagram illustrating the configuration of one ofthe plurality of conversion units 20 depicted in FIG. 2. The hardwareconfiguration of the conversion unit 20 can be implemented using acomputer. That is, the conversion unit 20 includes a processor 21 suchas a CPU, a storage unit 22 which stores an operating system (OS) 23 andother programs to be executed by the processor 21, a memory 25 intowhich programs to be executed by the processor 21 are loaded and inwhich data necessary for processing by the processor 21 are temporarilystored, and an interface 26 which receives the slit bitstream vk, one ofthe split bitstreams v1 to vn of the first format, from the splittingunit 10 and supplies to the connecting unit 30 the split bitstream sk ofthe second format converted from the stream vk.

The storage unit 22 stores a conversion program 24 which is executed bythe processor 21 and thereby causes the processor 21 to carry out theconversion process for converting the input split bitstream vk of thefirst format into the split bitstream sk of the second format. Thedetails of the bitstream conversion process implemented by theconversion program 24 will be described later.

FIG. 5 is a schematic diagram illustrating the configuration of theconnecting unit 30 depicted in FIG. 2. The hardware configuration of theconnecting unit 30 can be implemented using a computer. That is, theconnecting unit 30 includes a processor 31 such as a CPU, a storage unit32 which stores an operating system (OS) 33 and other programs to beexecuted by the processor 31, a memory 35 into which programs to beexecuted by the processor 31 are loaded and in which data necessary forprocessing by the processor 31 are temporarily stored, and an interface36 which receives the split bitstreams s1 to sn of the second formatfrom the respective conversion units 20 and outputs the bitstream S ofthe second format.

The storage unit 32 stores a connecting program 34 which is executed bythe processor 31 and thereby causes the processor 31 to carry out theconnecting process for connecting the plurality of split bitstreams s1to sn of the second format to one another and producing the bitstream Sof the second format having the same content as the bitstream V of thefirst format but different in format. The details of the bitstreamconnecting process implemented by the connecting program 34 will bedescribed later.

In the hardware configuration examples depicted in FIGS. 3 to 5, thesplitting unit 10, the conversion unit 20, and the connecting unit 30are each constructed from a separate computer, but the configuration ofthe bitstream conversion apparatus according to the present invention isnot limited to the illustrated one. Rather, any other configuration maybe employed, as long as the plurality of conversion units 20 a to 20 nare constructed so that the conversion processes can be performed in aparallel fashion using respectively separate processors.

FIG. 6 is a diagram illustrating a functional block diagram implementedby the splitting unit 10 depicted in FIG. 2. With the processor 11depicted in FIG. 3 executing the splitting program 14, the splittingunit 10 implements a splitting position determining unit 111 whichcarries out a splitting position determining process for determining thesplitting positions at which the bitstream V of the first format is tobe split into the plurality of split bitstreams v1 to vn, and a streamsplitting unit 112 which carries out the splitting process for splittingthe input bitstream V of the first format at the splitting positionsdetermined by the splitting position determining unit 111. How thebitstream is split by the splitting unit 10 will be described below.

When splitting the bitstream V of the first format into the plurality ofsplit bitstreams v1 to vn, the splitting unit 10 determines the startposition and end position of each of the split bitstreams v1 to vn so asto provide an overlapping portion between adjacent ones of the splitbitstreams v1 to vn, as earlier described with reference to FIG. 2. FIG.7 is a flowchart illustrating a first example of the bitstream splittingposition determining method according to the present invention. In theillustrated example, the bitstream splitting positions are determined sothat each of the split bitstreams v1 to vn contains substantially thesame number of key frames. The term “key frame” used in thisspecification refers to the frame generated without using interframepredictive coding. An I picture in a video stream encoded in an MPEGformat is an example of such a key frame.

In step S10, the splitting position determining unit 111 searches forall the image key frames contained in the bitstream V of the firstformat. Here, let zv denote the total number of image key frames and Nthe number of split bitstreams. Then, the splitting position determiningunit 111 determines the tentative start position tds of the i-th splitbitstream vi by taking a position somewhere between the position of the((i−1)×(zv/N)+1)th image key frame and the position of the((i−1)×(zv/N)+2)th image key frame (i is an integer between 1 and N).

Further, the splitting position determining unit 111 determines thetentative end position tde of the i-th split bitstream vi by taking aposition somewhere between the position of the (i×(zv/N)+1)th image keyframe and the position of the (i×(zv/N)+2)th image key frame.

In step S11, the splitting position determining unit 111 determines theimage stream start position tsv for each split bitstream vi. Here, thesplitting position determining unit 111 determines the start positiontsv by selecting the start position of the image key frame that appearsearlier than the time position located a predetermined time tav backfrom the tentative start position tds determined in step S10. Therelationship between the tentative start position tds, the startposition tsv, and the predetermined time tav is depicted in FIG. 8A.Each rectangle in FIGS. 8A and 8B indicates a frame, and each framelabeled A indicates an audio frame, while each frame labeled V indicatesan image frame. Each stippled frame indicates a key frame.

In step S12, the splitting position determining unit 111 determines theimage stream end position tev for each split bitstream vi. The splittingposition determining unit 111 determines the end position tev byselecting the position immediately preceding the start position of theimage key frame that appears later than the time position advanced by apredetermined time tbv from the tentative end position tde determined instep S10. The relationship between the tentative end position tde, theend position tev, and the predetermined time tbv is depicted in FIG. 8B.

By determining the image stream start and end positions as indicated insteps S11 and S12, the temporally adjacent split streams vi and vi+1have an overlapping portion at least equal to the predetermined timestav+tvb.

In step S13, the splitting position determining unit 111 determines theaudio stream start position tsa for each split bitstream vi. Thesplitting position determining unit 111 determines the audio streamstart position tsa by selecting the start position of the audio keyframe that appears earlier than the time position located apredetermined time taa back from the image stream start position tsvdetermined in step S11. The relationship between the respective startpositions tsa and tsv and the predetermined time taa is depicted in FIG.8A.

In step S14, the splitting position determining unit 111 determines theaudio stream end position tea for each split bitstream vi. The splittingposition determining unit 111 determines the audio stream end positiontea by selecting the start position of the audio key frame that appearslater than the time position advanced by a predetermined time tba fromthe image stream end position tev determined in step S12. Therelationship between the respective end positions tea and tev and thepredetermined time tba is depicted in FIG. 8B.

When the audio stream start and end positions are determined asindicated in steps S13 and S14, the audio stream in each split streamhas a longer overlapping portion than the image stream has. The reasonfor making the overlapping portion longer for the audio stream than thatfor the image stream is that the human perception is more sensitive to adisturbance in audio than to a disturbance in image. That is, by makingthe overlapping portion longer for the audio stream, it becomes possibleto make audio processing easier for connecting one audio stream toanother in a natural way.

The stream splitting unit 112 produces the split streams v1 to vn of thefirst format by segmenting the respective split bitstreams from thebitstream V of the first format based on the start and end positionsdetermined by the split position determining unit 111.

FIG. 9 is a flowchart illustrating a second example of the bitstreamsplitting position determining method according to the presentinvention. In the example illustrated in FIG. 9, the bitstream splittingpositions are determined so that each of the split bitstreams v1 to vnhas substantially the same data length (number of bits).

In step S20, the splitting position determining unit 111 determines the((i−1)×(L/N)+1)th bit position in the bitstream as the tentative startposition tds of the i-th split bitstream vi (i is an integer between 1and N). L denotes the total number of bits in the bitstream V, and N thenumber of bitstreams into which the bitstream V is split.

Further, the splitting position determining unit 111 determines the(i×(L/N)+1)th bit position in the bitstream as the tentative endposition tde of the i-th split bitstream vi. After that, the splittingposition determining unit 111 determines the start and end positions ofeach slit bitstream in the same manner as in steps S11 to S14 of theflowchart illustrated in FIG. 7.

FIG. 10 is a diagram illustrating a first example of a functional blockdiagram implemented by the conversion unit 20 depicted in FIG. 2. Withthe processor 21 depicted in FIG. 4 executing the conversion program 24,the conversion unit 20 implements an encoder 121 which converts thesplit bitstream vk of the first format (k is an integer between 1 and n)into the bitstream sk of the second format, a bit rate control unit 122which controls the bit rate as the encoder 121 generates the bitstreamsk of the second format, a buffer occupancy detection unit 124 whichdetects the occupancy of a stream buffer 123 which is a memory areaallocated as a speed smoothing memory within the memory 25, and anauxiliary information generating unit 125 which generates prescribedauxiliary information hk described hereinafter.

The auxiliary information hk is generated for each image key frame inthe bitstream sk of the second format. Since the split bitstreams v1 tovn of the first format to be converted by the conversion units 20 haveoverlapping portions tx1 to txn, the split bitstreams sk converted tothe second format also have overlapping portions. The auxiliaryinformation generating unit 125 may generate the auxiliary informationhk only for the image key frames contained in the overlapping portionsof the split bitstreams sk of the second format.

The auxiliary information hk may be information indicating the amount ofinformation produced when generating each corresponding image frame.Such information includes, for example, the amount of information hkfproduced when the encoder 121 processes the image frame, the occupancyhkv of the stream buffer 123 (hereinafter referred to as “bufferoccupancy”), etc.

The auxiliary information hk may also be image quality information hkqthat indicates the image quality of each corresponding image frame. Suchinformation includes, for example, information on the quantization stepsize used for encoding of the image frame by the encoder 121,information on coding errors, information on the distribution ofcoefficients, such as DCT coefficients, generated by intraframe coding,etc. When the image frame is encoded by dividing it into smallersegments such as macroblocks, the image quality information hkq may begenerated as a combination of numeric values one for each macroblock, ormay be generated by taking the average value of all the macroblockscontained in the image frame.

Further, the auxiliary information hk may be similarity metricinformation hks that provides a measure of similarity betweencorresponding image frames. Such information includes motion vectorinformation generated for each image frame.

The auxiliary information generating unit 125 generates the auxiliaryinformation hk by receiving the necessary information from the bufferoccupancy detection unit 124 and the encoder 121 in accordance with thecontents of the auxiliary information hk to be created.

FIG. 11 is a diagram illustrating a first example of a functional blockdiagram implemented by the connecting unit 30 depicted in FIG. 2. Withthe processor 31 depicted in FIG. 5 executing the connecting program 34,the connecting unit 30 implements a connecting frame positiondetermining unit 131 which, based on the auxiliary information h1 to hnreceived from the respective conversion units 20 a to 20 n, determinesthe connecting frame positions at which the split bitstreams s1 to sn ofthe second format, having overlapping portions and received from therespective conversion units 20 a to 20 n, are to be connected, anunnecessary data removing unit 132 which removes unnecessary data fromthe overlapping portions of the split bitstreams s1 to sn of the secondformat, and a data connecting unit 133 which produces the bitstream S ofthe second format by connecting together the split bitstreams s1 to snfrom which the unnecessary data have been removed.

The connecting frame position determining unit 131 determines, fromamong the positions along the overlapping portion of each pair oftemporally adjacent split bitstreams, the connecting frame position,i.e., the key frame position, at which the two split bitstreams are tobe connected.

For each of the split bitstreams s1 to sn of the second format, theunnecessary data removing unit 132 removes the portion between an end ofthe split bitstream and the connecting frame position, therebyeliminating the overlapping portions between the respective splitbitstreams s1 to sn.

The bitstream connecting method according to the present invention willbe described below. The following description is given by assuming thecase of connecting the temporally adjacent two split bitstreams sk andsk+1. FIG. 12 is a flowchart illustrating a first example of thebitstream connecting method according to the present invention.

In step S30, the connecting frame position determining unit 131calculates the difference Δh between the auxiliary information hskgenerated for the stream sk and the auxiliary information hsk+1generated for the stream sk+1.

The auxiliary information hsk and the auxiliary information hsk+1,respectively, are generated for each key frame contained in theoverlapping portion of the two streams sk and sk+1. In the followingdescription, the time t or position t of each frame in the bitstream Vof the first frame before splitting will be indicated by using the timeor position at which the frame is played back when the bitstream V isplayed back continuously from the start through to the end. The time orposition of each frame in the split bitstreams s1 to sn of the secondformat will also be indicated by using the time or position of thatframe in the source bitstream V of the first format. The auxiliaryinformation generated for a key frame at a given time t is designated ashsk(t) and hsk+1(t), respectively.

As previously described, the splitting unit 10 splits the bitstream V ofthe first format at key frame positions. Accordingly, when the splitbitstreams are converted by the conversion units 20 without changing thepositions of the image key frames or in such a manner that the frequencyof occurrence of image key frames in the split bitstreams s1 to sn ofthe second format becomes equal to an integral multiple of the frequencyof occurrence of image key frames in the split bitstreams vi to vn ofthe first format, the image key frames in the overlapping portion alignwith each other between the two split bitstreams sk and sk+1, asdepicted in FIG. 13.

The connecting frame position determining unit 131 calculates thedifference Δh(t) of the auxiliary information for each key frame. Forexample, when the auxiliary information hsk(t) includes the amount ofinformation hskf(t) generated when the key frame at time t wasgenerated, the buffer occupancy hskv(t) at time t, the key frame imagequality information hskq(t) at time t, and the key frame similaritymetric information hsks(t) at time t, and when the auxiliary informationhsk+1(t) includes the amount of information hsk+1f(t) generated when thekey frame at time t was generated, the buffer occupancy hsk+1v(t) attime t, the key frame image quality information hsk+1q(t) at time t, andthe key frame similarity metric information hsk+1s(t) at time t, thenthe difference Δh(t) may be calculated by the following equation (1).

Δh(t)=α×Δhf(t)+β×Δhv(t)+γ×Δhq(t)+η×Δhs(t)  (1)

Here, α, β, γ, and η are predetermined constants, and the variables aregiven as

Δhf(t)=|hskf(t)−hsk+1f(t)|  (2)

Δhv(t)=|hskv(t)−hsk+1v(t)|  (3)

Δhq(t)=|hskq(t)−hsk+1q(t)|  (4)

Δhs(t)=|hsks(t)−hsk+1s(t)|  (5)

In step S31, the connecting frame position determining unit 131 searchesfor a key frame position that minimizes the auxiliary informationdifference Δh(t) from among the key frame positions contained in theoverlapping portion of the bitstreams sk and sk+1, and takes the thusfound position (time) t as a candidate for the connecting frameposition.

In step S32, the connecting frame position determining unit 131determines whether, when the bitstreams sk and sk+1 are connected at theconnecting frame candidate position t, the resulting bitstream causes anoverflow or not, i.e., whether or not the amount of information at anygiven point on the bitstream exceeds a buffer occupancy upper limitvalue predefined for the buffer provided as a speed smoothing memory. Inthe following description, the phrase “causes an overflow” or “anoverflow occurs” may be used to refer to the situation where the amountof information at any given point on the bitstream exceeds and thusviolates the predefined upper limit value of the buffer occupancy.

The connecting frame position determining unit 131 can make the abovedetermination, for example, in the following manner. Denoting theconnecting position by a, then from the auxiliary information hsk(a)generated for the key frame at time a in the bitstream sk, theconnecting frame position determining unit 131 can determine the bufferoccupancy hskv(a) that occurred at time a when the bitstream sk wasencoded.

Further, based on the difference between the buffer occupancy hsk+1v(a)that occurred at time a when the bitstream sk+1 was encoded and thebuffer occupancy hsk+1v(b) that occurred at subsequent time b when thebitstream sk+1 was encoded, the connecting frame position determiningunit 131 can calculate the amount of information generated during theperiod from time a to time b when the bitstream sk+1 was encoded.

Accordingly, based on the buffer occupancy hskv(a) that occurred at timea when the bitstream sk was encoded and the buffer occupancies hsk+1v(a)and hsk+1v(b) that respectively occurred at times a and b when thebitstream sk+1 was encoded, the connecting frame position determiningunit 131 can calculate the buffer occupancy hv(b) that occurs at time bwhen the bitstreams sk and sk+1 are connected at time a.

The connecting frame position determining unit 131 determines whether anoverflow occurs or not by monitoring the thus calculated bufferoccupancy hv(b) for a predetermined period from time a and by checkingwhether the buffer occupancy hv(b) exceeds the predefined upper limitvalue during that period.

If it is determined in step S32 that an overflow does not occur, theconnecting frame position determining unit 131 determines the candidateposition checked in step S32 as the connecting frame position (stepS33). If it is determined in step S32 that an overflow occurs, theprocess proceeds to step S34.

In step S34, the connecting frame position determining unit 131 searchesfor the next connecting frame position candidate by searching for a keyframe position that minimizes Δh(t) from among the key frame positionscontained in the overlapping portion of the bitstreams sk and sk+1 butexcluding any key frame position previously chosen as the connectingframe position candidate.

When the next connecting frame position candidate is found (step S35),the candidate is checked in step S32, and the process from step S32 tostep S35 is repeated until no more candidate is found.

When the determination in step S32 has been made on all the key framescontained in the overlapping portion of the bitstreams sk and sk+1, butit has not been possible to find a position that does not cause anoverflow, then in step S36 the connecting frame position determiningunit 131 determines the connecting frame position by selecting the keyframe position where the difference between the buffer occupancy hskv(t)that occurred when the bitstream sk was encoded and the buffer occupancyhsk+1v(t) that occurred when the bitstream sk+1 was encoded is thesmallest, thus minimizing information loss due to the overflow.

FIG. 14 is a flowchart illustrating a second example of the bitstreamconnecting method according to the present invention. The method of thesecond example uses a larger number of candidate positions as thecandidates for the connecting frame position for connecting thebitstreams sk and sk+1. For this purpose, the conversion unit 20 notonly converts the split bitstreams vk and vk+1 of the first format intothe split bitstreams sk and sk+1 of the second format, but also convertsa sub-bitstream vk′1 contained in the overlapping portion of the splitbitstreams vk and vk+1 into a sub-bitstream sk′1 of the second format.The relationship between the bitstreams sk and sk+1 and thesub-bitstream sk′1 contained in their overlapping portion is depicted inFIG. 15.

For each pair of temporally successive bitstreams (sk and sk+1), aplurality of sub-bitstreams sk′1, sk′2, etc., contained in theoverlapping portion of the bitstreams may be generated. Such a pluralityof sub-bitstreams sk′1, sk′2, etc., may be generated, for example, byvarying the section or its length or the bit rate used when convertingto the second format.

In the following description, the position of each frame contained inthe sub-bitstream sk′1 will also be indicated by using the time at whichthe frame is played back in the source bitstream V of the first format.

The conversion unit 20 also generates auxiliary information hsk′1(t),hsk′2(t), etc. for each image key frame contained in the respectivesub-bitstreams sk′1, sk′2, etc.

In steps S40 to S45 depicted in FIG. 14, as in steps S30 to S35 in theconnecting method illustrated in FIG. 12, the connecting frame positiondetermining unit 131 searches for a connecting frame position at whichthe bitstreams sk and sk+1 are to be directly connected together.

If the connecting frame position at which the bitstreams sk and sk+1 areto be directly connected together has not been found, the connectingframe position determining unit 131 calculates in step S46 thedifference Δh1(t1) between the auxiliary information hsk(t1) for thebitstream sk and the auxiliary information hsk′1(t1) for thesub-bitstream sk′1 at a given key frame position t1 (depicted in FIG.15). Further, the connecting frame position determining unit 131calculates the difference Δh2(t2) between the auxiliary informationhsk′1(t2) for the sub-bitstream sk′1 and the auxiliary informationhsk+1(t2) for the bitstream sk+1 at a position t2 (depicted in FIG. 15)occurring later than the position t1. Here, Δh1(t1) and Δh2(t2) may becalculated in the same manner as the auxiliary information differenceΔh(t) between the bitstreams sk and sk+1 as calculated in accordancewith the earlier given equations (1) to (5).

In step S47, the connecting frame position determining unit 131 searchesfor positions such that Δh1(t1)+Δh2(t2) is minimized and such that anoverflow does not occur if the bitstream sk and the sub-bitstream sk′1is connected at position t1 and the sub-bitstream sk′1 and the bitstreamsk+1 are connected at position t2, and determines these positions t1 andt2 as the connecting frame positions.

If two or more sub-bitstreams sk′1, sk′2, etc. can be used, theconnecting frame positions that minimize the auxiliary informationdifference may be determined by repeating the steps S46 and S47 for eachsub-bitstream.

When the two frame positions t1 and t2 are determined as the connectingframe positions by using the sub-bitstream sk′1 as described above, thedata connecting unit 133 connects the bitstream sk and the sub-bitstreamsk′1 at the position t1 and the sub-bitstream sk′1 and the bitstreamsk+1 at the position t2, as illustrated in FIG. 15. The resulting streamS therefore contains the portion Δs1 preceding t1 in the bitstream sk,the portion Δs2 from t1 to t2 in the sub-bitstream sk′1, and the portionΔs3 succeeding t2 in the bitstream sk+1.

FIG. 16 is a diagram illustrating a second example of a functional blockdiagram implemented by the conversion unit 20 depicted in FIG. 2, andFIG. 17 is a diagram illustrating a second example of a functional blockdiagram implemented by the connecting unit 30 depicted in FIG. 2. In theillustrated example, if it is determined in step S35 in the flowchart ofFIG. 12 that an overflow occurs no matter what position is selected forconnecting the bitstreams sk and sk+1, the conversion from the firstformat to the second format is redone for the bitstream in theoverlapping portion of the bitstreams sk and sk+1.

For this purpose, if the occurrence of an overflow is detected whendetermining a connecting position in a given overlapping portion, theconnecting frame determining unit 131 depicted in FIG. 17 outputs errorinformation ek indicating the occurrence of the overflow to thecorresponding conversion unit 20 that performed the conversion of thebitstream in that given overlapping portion.

Suppose here that when determining the connecting position forconnecting the bitstreams sk and sk+1, the connecting frame positiondetermining unit 131 realizes that an overflow occurs no matter what keyframe in the overlapping portion P is selected as the connectingposition.

The error information ek issued in this case includes at least thebuffer occupancy hskv(a) and key frame image quality information hskq(a)generated for the key frame contained in the bitstream sk at the startposition a of the overlapping portion P, and the buffer occupancyhsk+1v(b) and key frame image quality information hsk+1q(b) generatedfor the key frame contained in the bitstream sk+1 at the end position bof the overlapping portion P.

When the error information ek is received, the conversion unit 20depicted in FIG. 16 redoes the encoding of the portion from a to b ofthe already input split bitstream of the first format. At this time, thebit rate control unit 122 performs overflow control so that theoccupancy of the stream buffer 123 at the end of the encoding processbecomes equal to the buffer occupancy hsk+1v(b) given by the errorinformation ek, assuming that the occupancy of the stream buffer 123 atthe beginning of the encoding process is equal to the buffer occupancyhskv(a) given by the error information ek.

Further, when redoing the conversion, the bit rate control unit 122performs bit rate control so that the image quality informationgenerated for the key frame at the start position a of the bitstream inthe overlapping portion P becomes equal to the image quality informationhskq(a) given by the error information ek and so that the image qualityinformation generated for the key frame at the end position b becomesequal to the image quality information hsk+1q(b) given by the errorinformation ek. That is, the image qualities at the start position a andend position b of the bitstream in the overlapping portion obtained byredoing the conversion are made to match the image qualities of therespective bitstreams sk and sk+1 of the second format, thereby makingthe variation in image quality at the connecting positions a and b lessnoticeable.

In this way, when performing the conversion of the bitstream in theoverlapping portion P, the bit rate of the bitstream encoded by theencoder 121 is controlled by the bit rate control unit 122 so that thedifference in the occupancy of the stream buffer 123 between the startposition a and end position b of the overlapping portion P becomes equalto (hsk+1v(b)−hskv(a)) and so that the image quality information at thestart position a of the overlapping portion P becomes equal to the imagequality information hskq(a) while, on the other hand, the image qualityinformation at the end position b becomes equal to the image qualityinformation hsk+1q(b).

The bit rate control unit 122 needs to perform control so that the imagequalities at the start and end positions a and b match the imagequalities specified by the respective image quality information hskq(a)and hsk+1q(b) given by the error information ek, while at the same time,controlling the amount of information occurring in the section betweenthe start and end positions a and b of the overlapping portion P. Theamount of information that occurs in the section from the start positiona to the end position b may be varied by continuously varying the imagequality along the way as depicted in FIG. 18 (that is, by varying thebit rate). In the illustrated example in which the two bitstreams areencoded by varying the image quality along the way as indicated by asemi-dashed line and a double-dashed line, respectively, the imagequalities at the start and end positions a and b become equal to therespective image quality information hskq(a) and hsk+1q(b), whereas theamount of information that occurs in the section from a to b differs.

While the present invention has been described in detail above withreference to the preferred embodiments, it should be understood by thoseskilled in the art that various modifications and changes can be made byanyone skilled in the art, and that all of such modifications andchanges that come within the range of the true spirit and purpose of thepresent invention fall within the scope of the present invention asdefined by the appended claims.

INDUSTRIAL APPLICABILITY

The present invention is applicable to media signal encoding techniquesfor compressing moving image signals and audio signals by exploitingspatial and temporal correlation inherently present in the moving imagesignals and temporal correlation inherently present in the audiosignals. The invention is particularly applicable to stream conversiontechniques for converting a bitstream containing a moving image streamand an audio stream from one format to another.

1. A bitstream conversion method for converting a bitstream of a firstformat, containing content data, into a bitstream of a second format,comprising: splitting the bitstream of the first format in timesequential fashion into a plurality of split bitstreams of the firstformat so as to provide an overlapping portion at each splittingposition; converting the plurality of split bitstreams of the firstformat in a parallel fashion into a plurality of split bitstreams of thesecond format; and connecting the plurality of split bitstreams of thesecond format to one another.
 2. A bitstream conversion method asclaimed in claim 1, wherein the bitstream of the first format contains amoving image stream and an audio stream, and wherein the overlappingportion at each splitting position of the audio stream is made to have alonger time length than the overlapping portion at each splittingposition of the moving image stream has.
 3. A bitstream conversionmethod as claimed in claim 1, wherein the bitstream of the first formatcontains a moving image stream, the method further comprising: for eachkey frame contained in the overlapping portion of the split bitstreamsof the second format, generating prescribed auxiliary information thatis referred to in order to select a connecting position at which toconnect together the split bitstreams of the second format; whenconnecting the plurality of split bitstreams of the second format to oneanother, determining each connecting frame position by selecting a keyframe position where the difference between the auxiliary informationgenerated for one stream and the auxiliary information generated for anensuing stream is the smallest; and connecting the one stream to theensuing stream at the connecting frame position.
 4. A bitstreamconversion method as claimed in claim 3, wherein the auxiliaryinformation includes at least information indicating picture quality ofthe key frame and the amount of information that occurs.
 5. A bitstreamconversion method as claimed in claim 3, wherein when the connectingframe position that satisfies a predetermined condition is not found,the conversion is redone for the bitstream in the overlapping portion.6. A bitstream conversion apparatus for converting a bitstream of afirst format, containing content data, into a bitstream of a secondformat, comprising: a splitting unit which splits the bitstream of thefirst format in time sequential fashion into a plurality of splitbitstreams of the first format; a plurality of conversion units whichconvert the plurality of split bitstreams of the first format into aplurality of split bitstreams of the second format; and a connectingunit which connects the plurality of split bitstreams of the secondformat to one another, and wherein the splitting unit splits thebitstream of the first format so as to provide an overlapping portion ateach splitting position.
 7. A bitstream conversion apparatus as claimedin claim 6, wherein the bitstream of the first format contains a movingimage stream and an audio stream, and wherein the splitting unit setsthe overlapping portion at each splitting position of the audio streamso as to have a longer time length than the overlapping portion at eachsplitting position of the moving image stream has.
 8. A bitstreamconversion apparatus as claimed in claim 6, wherein the bitstream of thefirst format contains a moving image stream, and wherein each of theconversion units includes an auxiliary information generating unitwhich, for each key frame contained in the overlapping portion of thesplit bitstreams of the second format, generates prescribed auxiliaryinformation that is referred to in order to select a connecting positionat which to connect together the split bitstreams of the second format,and the connecting unit includes: a connecting frame positiondetermining unit which determines a connecting frame position forconnecting one stream to an ensuing stream by selecting a key frameposition where the difference between the auxiliary informationgenerated for the one stream and the auxiliary information generated forthe ensuing stream is the smallest; and a data connecting unit whichconnects the one stream to the ensuing stream at the connecting frameposition.
 9. A bitstream conversion apparatus as claimed in claim 8,wherein the auxiliary information includes at least informationindicating picture quality of each frame and the amount of informationthat occurs.
 10. A bitstream conversion apparatus as claimed in claim 8,wherein when the connecting frame position that satisfies apredetermined condition is not found, the conversion unit redoes theconversion for the bitstream in the overlapping portion.
 11. Acomputer-readable storage medium storing a bitstream splitting programwhich, when executed by a computer for bitstream splitting process,causes the computer for bitstream splitting process to execute a processcomprising: splitting a bitstream of a first format, containing contentdata, in time sequential fashion into a plurality of split bitstreams ofthe first format which are to be converted into a plurality of splitbitstreams of the second format in a parallel fashion by a plurality ofconversion units, so as to provide an overlapping portion at eachsplitting position.
 12. The computer-readable storage medium as claimedin claim 11, wherein the bitstream of the first format contains a movingimage stream and an audio stream, and the overlapping portion at eachsplitting position of the audio stream is made to have a longer timelength than the overlapping portion at each splitting position of themoving image stream has.
 13. A computer-readable storage medium storinga bitstream conversion program for converting the split bitstream of thefirst format containing at least a moving image stream, provided by thebitstream splitting program stored in the computer-readable storagemedium of claim 11, when the bitstream conversion program is executed bya computer for bitstream conversion process, the bitstream conversionprogram causing the computer for bitstream conversion process to executea process comprising: converting the split bitstream of the first formatinto the split bitstream of the second format, and generating, for eachkey frame contained in the overlapping portion of the split bitstream ofthe second format, prescribed auxiliary information that is referred toin order to select a connecting position at which to connect togetherthe split bitstreams of the second format.
 14. The computer-readablestorage medium as claimed in claim 13, wherein the auxiliary informationincludes at least information indicating picture quality of the keyframe and the amount of information that occurs.
 15. A computer-readablestorage medium storing a bitstream connecting program for connecting theplurality of split bitstreams of the second format to one another,provided by the bitstream conversion program stored in thecomputer-readable storage medium of claim 13, when the bitstreamconnecting program is executed by a computer for bitstream connectingprocess, the bitstream connecting program causing the computer forbitstream connecting process to execute a process comprising:determining a connecting position for connecting one stream to anensuing stream by selecting a key frame position where the differencebetween the auxiliary information generated for the one stream and theauxiliary information generated for the ensuing stream is the smallest;and connecting the one stream to the ensuing stream at the thusdetermined connecting position.
 16. The computer-readable storage mediumstoring the bitstream conversion program providing split bitstream ofthe second format to the bitstream connecting program stored in thecomputer-readable storage medium of claim 15, the bitstream conversionprogram causing the computer for bitstream conversion process to executethe process further comprising: reconverting the bitstream in theoverlapping portion when the computer for bitstream connecting processon which the bitstream connecting program is run is unable to find theconnecting frame position that satisfies a predetermined condition. 17.A bitstream conversion method as claimed in claim 4, wherein when theconnecting frame position that satisfies a predetermined condition isnot found, the conversion is redone for the bitstream in the overlappingportion.
 18. A bitstream conversion apparatus as claimed in claim 9,wherein when the connecting frame position that satisfies apredetermined condition is not found, the conversion unit redoes theconversion for the bitstream in the overlapping portion.